fix unbound variable

Author: ckrause

loda/runtime/interpreter.py

@@ -100,7 +100,7 @@ def __run(self, p: Program, mem: dict) -> int:
 
         # check for loops with fragments
         if any(op.type == Operation.Type.LPB and op.source != Operand(Operand.Type.CONSTANT, 1) for op in ops):
-            self.__raise("unsupported operation: {}".format(op))
+            self.__raise("fragments not supported")
 
         # check for empty program
         if len(ops) == 0: